令 x1=y1+y2,x2=y1-y2,x3=y3
则 f = (y1)^2-(y2)^2+2y1y3+2y2y3-4y1y3+4y2y3
= (y1)^2-(y2)^2-2y1y3+6y2y3
= (y1-3y2-y3)^2-10(y2)^2-(y3)^2
= (z1)^2-10(z2)^2-(z3)^2
其中 z1=y1-3y2-y3=-x1+2x2-x3
z2=y2=(x1-x2)/2
z3=y3=x3.